Method for removing surface coatings

ABSTRACT

A method of removing a coating, such as paint, varnish, biological growth or grime, from a surface, the method comprising selecting a suitable particulate solid having a particular, size of from 150-250 mm and a fluid carrier to form a spray mixture and spraying the mixture as a jet spray so as to impact and at least partially remove the coating. The hardness of the particulate solid is less than 8.0 on the Moh scale. The pressure applied to the spray mixture to generate the spray is from 3.sub.×10.sup.5 to 1.5.sub.×10.sup.6 Nm.sup.−2. An apparatus comprising a blasting pot ( 1 ) and a compressor ( 2 ) to generate the spray mixture and propel it from a nozzle ( 5 ) is also disclosed. Heating may be applied to the carrier, either prior to or when mixing with the particulate solid.

RELATED APPLICATIONS

This application is a continuation of currently pending U.S. patent application Ser. No. 10/421,605 filed Apr. 23, 2003, which is a continuation of U.S. Ser. No. 09/857,513, now U.S. Pat. No. 6,609,955; which claims the benefit of the filing date of International Patent Application No. PCT/GB99/04108, which claims the benefit of Great Britain Patent Application Nos. 9826683 and 9827214; of which the contents of these documents are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to removing coatings from a surface and more particularly to removing paint, varnish or biological growth from the outer hull of a boat.

BACKGROUND OF THE INVENTION

The removal of a layer or layers from a surface by impacting an abrasive material against the layer of layers is well known. For example, grit or sand-blasting has been used for many years to clean stone buildings, painted metal surfaces such as railings and superstructures including oil rigs. The particles of grit or sand are mobilized by means of a carrier fluid, normally air or water.

The commonly used methods suffer from the drawback that damage is often caused to the material beneath those layers being removed. This is especially true where the methods are employed to remove surfaces from a relatively soft material such as wood or fibre glass. In particular, where fibre glass is being cleared, damage can be caused to the gel coat layer. The problem of damages caused is particularly acute where, for example, antique wooden objects are being cleaned or where the surface is part of a boat.

German patent application DE 19522001 (MINERALIEN WERKE) discloses the use of a mixture of solids, one of the solids having a higher density than the other, in order to clean and treat sensitive or polished surfaces such as brick or marble.

Where water is used as the carrier fluid, then its consumption using conventional methods is often quite high. Where there is a ready supply of water high consumption may not be a problem but where, due to the remoteness of a source, the water needs to be transported to the object to be cleaned, minimization of water consumption would be advantageous.

It is an object of the present invention to provide a method which alleviates the above disadvantages. It is a further object of the present invention to minimise the usage of the carrier fluid when said fluid is a liquid.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of removing a coating such as paint or varnish from a surface, the method comprising:

-   -   (i) selecting a particulate solid suitable for removing the         coating from the surface, the particulate solid having a         particle size from 150 to 250 μm;     -   (ii) selecting a fluid as a carrier for the particulate solid;     -   (iii) distributing the particulate solid in the fluid to form a         spray mixture;     -   (iv) generating a pressurised jet of the spray mixture;     -   (v) impacting onto a coating, the pressurised jet of spray         mixture to removing the coating.

Preferably, the particle size is from 170 to 190 μm.

The hardness of the particulate solid is preferably less than 8.0 on the Moh scale. It is particularly preferable for the hardness to be 6.0 to 7.0 on the Moh scale. The preferred particulate solid is olivine.

In one arrangement, the method includes heating the carrier fluid.

The fluid is preferably a liquid. It is particularly preferable that the liquid used is water.

The temperature of the liquid is preferably maintained below 50 C. It is particularly preferable that the liquid is heated to a temperature between 25 and 40 C.

The solid to liquid ratio in the jet spray is preferably approximately 2 to 1 volume for volume.

The jet advantageously is directed to impact the coating at an angle of approximately 45°.

In used, the jet is removed—preferably in a circular motion—back and forth across the coating to be removed.

The pressure of the jet is advantageously from 1×10⁵ to 1.5×10⁶ Nm⁻² and preferably from 4×10⁵ to 1×10⁶ Nm⁻².

BRIEF DESCRIPTON OF THE DRAWINGS

The present invention will now be described more particularly with reference to the accompanying drawing which shows, by way of example only, apparatus for removing a coating from a surface in accordance with the method of the invention. In the drawing:

FIG. 1 is a diagrammatical view of the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, an apparatus, suitable for use in the method detailed below, comprises a blasting pot 1 and a compressor 2. Compressed air is passed from the compressor 2, via an inlet valve 3 to the blasting pot 1. Water is supplied to the blasting pot via an inlet pipe 6. The blasting pot 1 also comprises an outlet pipe 7. The outer pipe 7 has as its distal end a nozzle 5. Flow of material to the nozzle 5 is controlled by means of outlet valve 4.

In the method accordingly to the invention a spray mixture of olivine and water from the domestic supply, at ambient temperature, is charged to the blasting pot 1. Compressed air at a pressure of approximately 7×10⁵ Nm⁻² from the compressor 2 is then passed through the inlet valve 3 and pressurises the blasting pot 1 up to approximately 12×10⁵ Nm⁻².

When suitable pressure has been reached in the pot 1, the pressure can be released when required by opening the outlet valve 4 which is attached to the nozzle 5. The nozzle 5 is approximately 15 cm long with an outlet diameter of approximately 1.9 cm. The excess pressure forces the spray mixture of olivine and water out of the pot 1 and through the nozzle 5 at a pressure, often called the blast pressure, of approximately 6×10⁵ Nm⁻²

When the spray mixture is ejected through nozzle 5 an directed against the surface coating as described hereinbelow, it acts to abrade the coating and remove it whilst leaving the surface beneath the coating relatively undamaged and ready to be treated or for a new coating to be applied. When necessary, a constant water feed may be introduced via the inlet pipe 6.

The nozzle 5 can either be directed by hand or remotely. When directed by hand, the nozzle is held such as to deliver the abrading spray mixture at an angle to the coated surface. The angle is usually approximately 45°.

The distance the nozzle is held from the surface will vary according to the conditions under which spraying is being carried out, the mixture being sprayed and the coating being removed. A distance of approximately 50 cm has been found to give good results for a variety of coatings.

Although a coating can be removed by simply passing the nozzle across the surface in a single sweeping action, a number of passes could be carried out, each subtending the same angle to the surface, or subtending a different angle.

In addition, a circular motion may also be imparted to the jet spray to improve coating removal. The circular motion can be imparted manually, by the action of a jet spray or by mechanical means. For example, the action of the spray leaving the nozzle is used to induce motion in the nozzle, by giving the direction of the spray a radial component relative to the longitudinal axis of the nozzle. Alternatively, a small motor is used to move the nozzle in a circular motion.

For ease of operation, the outlet valve 4 is conveniently situated adjacent to the nozzle but can be remote from it. The nozzle can have different forms to deliver particular jet sprays where required. The width of the outlet of the nozzle should be wide enough to prevent clogging, and narrow enough to allow concentration of the force delivered by the spray onto a small enough area to be effective. For hand-held nozzles the outlet is typically 1¼ to 2½ cm in diameter.

The inlet air pressure admitted through value 3 is typically 6×10⁵ to 10×10⁵ Nm⁻². Although compressed air supplied by an on-site compressor will usually be most convenient, air or other gases supplied in pressurised cylinders can also be used, for example, where no power source for a compressor is available. The pressure built up inside the pot 1, prior to spraying is typically less than 20×10⁵ Nm² and is normally less than 14×10⁵ Nm². The blast pressure can be up to 12×10⁵ Nm⁻² but can be as low as 4×10⁵ Nm². The pressure used will depend very much on the coating being removed, and on the nature of the surface which is coated.

During use, the initial pressure built up in the pot will drop back from its initial value, perhaps down to approximately 3×10⁵ Nm⁻².

The composition of the jet spray delivered can be varied by alternation of the rate of water addition to the pot, but can also be varied by changing the operating pressures. The composition can thus be adjusted to suit the nature of the coating material being removed, and the surface which it coats. A typical composition will be between approximately 1:1 and 3:1 of particulate solid to liquid.

As alternatives or in addition to olivine (also known as forsterite)—which depending on its composition has a Moh hardness of between 6.5 to 7—other minerals such as andalusite, spodumene, diaspore, congolite, spessartine and adesine may also be used. In addition, man-made materials in the form of a particulate solid of the requisite hardness range may also be used.

The particulate solid can have particle size of 60 to 100 mesh. It has been found that if the particles are too large, then they can cause damage to the surface itself, rather than simply removing the coating. A mixture of particles having differing mesh sizes could also be used.

The water used can, it addition to coming from the domestic supply, also be fresh water or sea water. If sea water is used then the surface should preferably be rinsed off with domestic supply water, prior to its being re-coated. As alternatives to or in addition to water, certain organic or inorganic solves may also be employed. Examples of solvents which can be employed are alkyl alcohols, such as ethanol, propanol, iso-propanol, ethylene glycol or propylene glycol. Other solvents which may be contemplated include acetone, butanome and sulpholane. Especially suitable are those which may have a solublising or swelling effect on the surface coating being removed, thus rendering it more easily removable. When necessary, suitable measures will need to be taken to protect the operator and the environment from these solvents.

In addition to liquids as described above, other fluids may be employed, either partially or fully in their place. Suitable examples of fluids which may be used include air or nitrogen.

The water supplied from a domestic or external source is normally provided at a temperature of below 20 C. Where necessary however it may be heated up to about 40 C. Heating the water to a temperature of between 25 to 40 C has been found to reduce water consumption. The heating may be accomplished by means of an independent heating element mounted within the blasting pot or alternatively to the water inlet supply. As an alternative, where a petrol or diesel powered generator is used to operate for example a compressor to produce compressed air, then the exhaust pipe can pass through the water, on its way to the gases being vented, and the heat from the exhaust can be utilised to raise the temperature of the water.

The apparatus used can conveniently be bolted to a trailer or other suitable transporting vehicle to enable it to be taken more easily to where it is required. This will allow for example, a supply of water for spraying to be taken, where it would otherwise be difficult to obtain.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible with the scope of the appended claims. 

1. An apparatus for removing a coating from a surface, the apparatus comprising: a blasting pot including an inlet pipe and an outlet pipe including a nozzle, the blasting pot holds a mixture of a particulate solid and a fluid, wherein the particulate solid includes a particle size of 150 to 250 μm and a hardness less than 8.0 on the Moh scale; a compressor for pressurizing the blasting pot; and, an inlet valve operatively connected between the blasting pot and the compressor; wherein compressed air is provided to the blasting pot via the compressor, wherein the mixture is sprayed from the nozzle at an approximate pressure of between 4×10⁵ to 1×10⁶ Nm⁻².
 2. The apparatus of claim 1 further including a means for heating the fluid, wherein the fluid is heated to a temperature between approximately 25 to 40° C.
 3. The apparatus of claim 2 wherein the means for heating the fluid is a heating element within the blasting pot.
 4. The apparatus of claim 2 wherein the means for heating the fluid is a heating element operatively attached to a fluid inlet supply, which is connected to the blasting pot.
 5. The apparatus of claim 1 wherein the fluid includes water, air, or nitrogen.
 6. The apparatus of claim 1 wherein the fluid includes organic or inorganic solvents.
 7. The apparatus of claim 6 wherein the organic or inorganic solvents include alkyl alcohols.
 8. The apparatus of claim 7 wherein the alkyl alcohol includes ethanol, propanol, iso-propanol, ethylene glycol, or propylene glycol.
 9. The apparatus of claim 6 wherein the organic or inorganic solvents include acetone, butatone, or sulpholane.
 10. The apparatus of claim 1 wherein the particulate solid includes olivine, andalusite, spodumene, diaspore, congolite, spessartine, or andesine.
 11. The apparatus of claim 1 wherein the particulate solid includes a man-made material having the requisite hardness. 